Noiseless sound reproducing system



'OC- 20 1942 B. KREUYZER l 252995393 .f

NOISELESS SOUND REPRODUCING SYSTEM Filed April 30, 1941 -60 .rf/.M LEVEL N11/9 Win45" kk INVENTOR- Mw'o/v Kaff/zee,

ATTORNEYS.

Patented Get. 20, 1942 NOISELESS SOUND REPRODUCING SYSTEM- Barton Kreuzer, Los Angeles, Calif., 'assignor to Radio Corporation of America, a corporation of Delaware Application April 30, 1941, Serial No. 391,050

9 Claims. (Cl. 179-1003) This invention relates to sound reproducing and recording apparatus and particularly to a method of and system for the rerecording and reproduction of sound.

It has been found that sound tracks appear to have a greater overall quietness and provide a better overall quality if the noise is suppressed or eliminated. between the actual sound periods. For instance, in the case of dialogue there are periods of original silence between spoken phrases or sentences and between the conversations of two or more persons. If during these periods th reproduction of the sound record can be made absolutely or substantially silent, the quality of the sound is not only enhanced, but the dramatic effect or expression of the dialogue is improved. This means the substantial elimination of all sound at the loudspeakers usually caused by ground-noise. Such ground-noise may be produced by the lack of noise reduction on the lm or by dirty or scratched lm or by film graininess.

Noise reduction systems are well known wherein the signal to noise ratio is maintained high even during low amplitudes of the signal.v In variable area systems this is' accomplished by eliminating the light from the reproducing cell by blackening the areas of the sound track not occupied by the sound modulations, while in variable density systems, the average density of the film is increased as the signal amplitudes decrease. It is obvious, however, that these types of noiseI reduction systems cannot introduce absolutequiet at time of no signal because complete noise reduction is not practical between sig-4 nals since a certain safety margin must be retained on the lm to avoid the loss of the very low modulations and to preventclipping. Thus, there is a certain noise level present at such times. However, even ifsuch noise reduction systems did function to completely eliminate noise from the loudspeakers, it would not prevent extraneous light getting through to the photoelectric cell during the silent or no-signal periods due to dirt or scratches on the film.

The principal'object of the present invention, therefore, is to improve the rerecording and reproduction of sound from sound records. L,

Another object Aof the invention isto eliminate the reproduction of sound during silent yperiods of arecord.

A further object of the'invention is to control the reproduction of sound in accordance with the signal level.

A further object of the invention is to reduce 55 the efliciency and .eiectiveness ofa photoelectric cell during times of no-signal modulation.

A furtherobject of the invention is to increase the effectiveness of a photoelectric cell during the impression thereon of light modulations to be reproduced.

Although the novel features which are believed to be characteristic of this invention are pointed 'out with particularity in the appended claims, the

manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjuncticn with the accompanying drawing forming a part thereof in which Y Fig. l is a diagrammatic-schematic diagram of a rerecording system embodying the invention; and

Figure 2 is a graph illustrating the operation of the system of Fig. l.

electric cell I0, conductor I5, primary of audio coupling transformer I'I shunted by a resistance I8, control resistance 20, to ground 2|. An-alter nating -current circuit for the photoelectric cell I0 includes the by-pass condenser 22.

The output of thephotoelectric cell I0 is impressed upon a preamplifier 25 through the transformer I 1 and then upon the `sound recorder 9 and monitoring loudspeaker I2. Bridged across the transmission circuit from the preamplifier is l a bridgingamplifier 2l feeding a variable attenuator 28 and a rectier amplifier unit 29. The bridging amplifier 21 could also be ccnnected across the input to amplifier 25. The load of'the unit 29 is the resistance 20 connected over conductors 3| and 32 to respective grounds 2I and 33;

By the4 use ofv the bridging amplifier. 2'II and Variable attenuator 28, the desired irputlevcligs impressed on the rectier .unit 29, '.l'lie llinit` 29 may be theusual type of noise reduction unit employed in sound recording in wrioh a rectier and lter unit is employed, a typical uni'.- being an RCA N11-3218. This unit can be cf the exponential type wherein the-ratio of signal levrl to D. C. output varies exponentially to provide the proper input-'output relationship.

` the battery or other voltage supply l2. insures that a small voltage of the correct polarity l characteristic.

The output of the unit 29, as 'mentioned above,

varies the voltage across the resistance 20 in aci cordance with the. signal level. The adjustment i of the system is such that when no signal is im- .3 pressed upon the photocell I0, the output of the unit 29 produces ay voltage acrossvthe resistance 20 slightly less than the polarizing voltage of This is supplied vto the photocell I0. Thus, during a no-signal condition the transmission eciency of the photocell I is extremely low. However,

as soon as the signal on the -lm'is impressed upon the photoelectric .cell i0, sucient signal passes to the unit 29 to reduce the voltage across the resistance 20 and thus increase the voltage g upon the photocell l0.

\ The increase and decrease in voltage acrossI the t resistance 20, and, consequently, the increase and decrease in voltage upon the photoelectric cell I0 are extremely rapid over a small range vof low signal levels, after which these voltages remain substantially' constant. This action may b e explained by referring to Fig. 2 wherein iilm level is plotted against the gain or amplification of the riocls either from iilm rerecorded orreproduced currents by said electrical currents whereby light variations below a predetermined level are substantially eliminated from said current-toesound translating means. f

`2. A sound reproducing system in accordance i with claim 1 in which said last-mentioned means entire system.l The solid curve ABC shows that between the lm range of -60 db. and fulltrack modulation taken as zero db., the gain is not only higher than below` the- -60 db. level,

' but is substantiallyconstant. The general shape of this curve is produced by the exponential acincludes an electronic translator of said currents having a characteristic substantially ilat over a.

predetermined range of amplitudes and comparatively steep over a lower range of amplitudes.

3. The method` of rerecording a sound record from a lm comprising generating electrical currentsfrom a light beam varying in substantial accordance with the ratio of variation of a sound tionof the noise reduction unit, this action'ate tenuating-currents produced by any light variations'causedby dirt or scratches appearing upon the sound track .area of the film during periods of silence or periods; of low modulations below -60 db. In the art of sound recording it has been found that modulations lying below this film level may be substantially eliminated inasmuch as any desired signal modulations to be recorded arev maintained above this point so that the iilm signal rangel being utilized lies between -60 to Thus, bythe use of the invention in a rerecording system as illustrated the new sound track will not have thereon any undesired modulations caused bydirt. scratches or the graininess, of the iilm being rerecorded between the true sound periods. When the invention is used in a vsystem reproducing a fllm not rerecorded with the in- It is obvious, of course, that a silent track may be reproduced with the invention.

vention, then the same result isdirectly' obtained.

In dotted lines interposed between thebridging amplifier 21 and variable attenuator 28,; is shown a volume limiter 35, this limitervbeing usable inthe circuit if it is desired to iiatten or maintain the system gain more nearlyconstant between the #-60 db. level point and the zero db.

, level point, as shown by the dotted lines BD in Fig. 2. By the useoi ayolume limiter, such as an RCA broadcast limiter 96A, the combination the use of an exponential rectiiierfampliiier at point E, thus limiting the system' to a con- 1 characteristic ABD is obtainable. The dot-and- 1 dash characteristic ABEF is obtainable by combination' adjusted to run out' of' current stent gain between points E and F. A linear recv tier-ampliiler unit'may also be adjusted to run out of current at .point B, providing the ABD Thus,v almost any desired operating characteristic is obtainable.

Withthe above-described invention, therefore, it is possible to obtain substantially complete silence at the loudspeakers during no-si'gnai Pe record and varying the eiiiciency of generation of said currents from said light beam to substantially prevent the generation of currents during the periods between occurrences of said sound record on said lm.

4. The method of selectively varying the generation of currents vfrom light variations comprising generating electrical currents in direct proportion to said light variations at a predetermined eiiiciency over a predetermined range of amplitudes of light variations and decreasing the efficiency of generation of electrical currents to substantially eliminate the generation thereof from light variations having amplitudes below a predetermined amplitude. v v 5. The method of selectively reproducing a sound record having modulations of different amplitudes comprising scanning said record to ob tain light variations in accordance therewith,

generating electrical currents from all light variations above a certain amplitude at a substantially constant efiiciency, generating electrical currents from all light variations below said certain am- `plitude at a lower eillciency, and translating said currents into sound waves.

. 6. The method in accordance with claim 5 in which the eiiiciency of generation of said modula, f 'tions below saidcertain amplitude decreases with decreases in amplitude of said modulations.-

1.-A sound reproducing system comprising a sound record having modulations of different amplitudes and periods of silence therein, means for lobtaining light variations corresponding to said record, means for generating electrical currents withcertain of said light variations,-means for translating said currents into sound waves, and

means connected intermediate vsaid generating;

means and said translating means for deriving a voltage corresponding to the average value `of said currents for varying the efficiency of generation of .said electrical currents in accordance with the amplitudes of the modulations of .said record.

8. A sound reproducing system comprising a'.

sound record having modulations'of different amplitudes and periods of silence therein, means for translating' said modulations into electrical c urrents, means for translating said currents into sound waves, and means connected to and inter- A 2,299,398 mediate said rst-mentioned translating means and said second-mentioned translating means for varying the eiiiciency of said mst-mentioned translating means in accordance with the amplitudes of the modulations of said record, said ilrstmentioned translating means comprising a photocell and said last-mentioned means including means for varying the polarizing voltage on said cell.

9. A sound reproducing system comprising a sound record having modulations of different amplitudes and periods of silence therein, means for translating said modulations into electrical currents, means for translating said currents into sound waves, and means connected to and intermediate lsaid mst-mentioned translating means and said Jsecond-mentioned translating means for varying the efllciency of said rst-mentioned of said modulations and maintaining said polarizing voltage substantially constant for changes of amplitude of said modulations in the higher amplitude range of modulations.

BARTON KREUZER. 

